Category Archives: Mechanical Heliostat

All the post connected with the mechanical heliostat

Manual heliostat

The first steps towards the manual heliostat. A couple of square meters for test purposes must be doable by hand. East west the mirrors can simple pivot around their base and up and down is accomplished by a screw-thread.

backreflector
The back reflector
backside manual heliostats
backside manual heliostats
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Little 30 x 30 cm tile mirrors
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near the focus

With 6 mirrors the open test cell reached already 80 degrees Celsius. With the back reflector the temperature went over the 100 degrees Celsius.

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How high is de wind pressure on a Heliostat?

In order to design a heliostat we have to understand the forces that will act on the structure. During a storm it’s more than likely cloudy so the heliostat only has to work in low wind-speeds. Wind-speeds in m/s x 3.6 gives the wind-speed in km/h, 20m/s = 72 km/h. which gives a pressure of 0.5 X 1.25 x 20² = 250 N/m².

Wind speed (m/s) Wind pressure (N/m2)
Lower limit Upper limit Upper limit
0 0.2 0.03
0.3 1.5 1.4
1.6 3.3 6.8
3.4 5.4 18
5.5 7.9 39
8 10.7 72
10.8 13.8 119
13.9 17.1 183
17.2 20.7 268
20.8 24.4 372
24.5 28.4 504
28.5 32.5 660
32.6 > 660
The wind pressure can be approximated by: Pressure = ½ x (density of air) x (wind speed)² x (shape factor)

  • The density of air is about 1.25 kg/m³.
  • The shape factor (drag coefficient) depends on the shape of the body. It has order of magnitude 1 and is dimension less.
  • The wind speed must be expressed in m/s. In that case the pressure has units kg/m/s², i.e. N/m².

Probably the most important environmental design criterion that must be met by a heliostat design is the wind condition. Typical requirements may be for the heliostat to meet its operating requirements in a 12 m/s wind, to survive a 22 m/s wind, and to continue to operate or move to the stow position in a 40 m/s wind (a position usually horizontal with mirrors face-up or face-down). Also, the ability to survive hail is important for any flat surface exposed to the elements. A typical hail survival criterion is 19 mm diameter hailstones impinging at 20 m/s. source: the power of the sun

One of the ten little test mirros 30×30 cm

 

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Escapement heliostat

Which mechanism to use to move the heliostat.

There are a number of different escapement mechanisms invented. From very delicate to very robust. One sturdy one is gravity escapement used in bell-towers.

Key to thwww.trin.cam.ac.uk/clock/escapement.htmle animation: The two “gravity arms” are coloured blue and red. The two three-legged escape wheels are also coloured blue and red. They work in two parallel planes so that the blue wheel only impacts the locking block on the blue arm and the red wheel only impacts the red arm. In a real escapement these impacts give rise to loud audible “ticks” and these are indicated by the appearance of a * beside the locking blocks.
The three black lifting pins are key to the operation of the escapement. They cause the weighted gravity arms to be raised by an amount indicated by the pair of parallel lines on each side of the escapement. This gain in potential energy is the energy given to the pendulum on each cycle. For the Trinity Clock a mass of around 50 grams is lifted through 3mm each 1.5 seconds – which works out to 1mW of power. The driving power from the falling weight is about 12mW, so there is a substantial excess of power used to drive the escapement. Much of this energy is dissipated in the acceleration and deceleration of the frictional “fly” attached to the escape wheels.

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